Anim Preview Editor

The Anim Preview Editor found inside of Animation Blueprints
displays all the Variables
and Variable categories associated with an Animation Blueprint.
When you open any Animation Blueprint, you can find the Anim Preview Editor in the lower-right corner of the Animation Blueprint Editor.
The Anim Preview Editor not only displays the variables associated with the Animation Blueprint, it allows you to adjust the values to see how it will affect any logic which may lead to changes in a character's animation state behavior.

Click image for full view.

Also included by default in this location on a separate docked tab is the Asset Browser
window.

Variables created inside an Animation Blueprint can be assigned to categories for organizational purposes (a Default category is used if no category is selected).

You can type in a name for a category for the variable to be assigned to or select a previously created category from the drop-down menu.

Anim Preview Editor Modes

The Anim Preview Editor is split into two modes: Edit Preview and Edit Defaults

Edit Preview

The default option for the Anim Preview Editor is the Edit Preview mode which allows you alter variables that affect the preview Skeletal Mesh only.

Above, the option IsInAir? has been set to True and the preview mesh (based on the Animation Blueprint's logic) enters a falling state.
Whenever you make changes inside the Anim Preview Editor and you are on the Edit Preview mode, those changes only affect the preview mesh and the changes you make will not be saved unless you click the Apply button which will take your values and assign them to the new default values.